This invention relates to a semiconductor photodiode more particularly a semiconductor junction photodiode wherein a sufficiently high reverse bias voltage is applied across the junction for the purpose of utilizing the avalanche multiplication phenomena of the carriers and a method of manufacturing such photodiode.
In recent years various technology have been developed for effecting optical fiber transmission. With regard to an optical detector utilized to detect optical signals transmitted through optical fibers, it has been strongly desired to obtain an improved semiconductor photodiode which is suitable to be compatible with the optical fibers small size and having a high sensitivity and high operating speed. An avalanche photodiode (APD) made up of silicon has been used for fulfilling those requirements. However, since it has been found that a wavelength band of from 0.8 to 0.9 .mu.m is suitable for the low loss region of the optical fibers as well as the oscillation possible region of AlGaAs semiconductor lasers, the use of a conventional Si-APD presents the following problems. More particularly, the absorption coefficient of Si decreases in the wavelength bands from 0.8 to 0.9 .mu.m and received light reaches deep portions in the APD. Since the photocarriers formed by the light arriving at the deep portions migrate by diffusion, they arrive at the junction with a time lag thus decreasing response speed. Such decrease in the absorption coefficient decreases the quantum efficiency. For this reason according to a prior art design it has been necessary to apply high bias voltages of 200 to 400 volts, for example, in order to improve the response speed and the quantum efficiency. Further, after the electric field dependency of the ionization coefficient upon the electrons and holes in silicon has been clearly analyzed a method of designing an avalanche region utilizing such data has become possible. Thus, with reference to silicon, when the avalanche multiplication is effected in a low electric field region, the ionization coefficient ratio of the holes and electrons decrease thereby decreasing the noise formed during the avalanche multiplication process. Such expectation for low noise resulted in a strong request for improvement of the quantum efficiency, response speed and the operating voltage.
Considering the above described APD, when the width of the depletion layer formed in the interior of the diode is increased it is possible to correspondingly increase the quantum efficiency, that is the optical to electrical signal conversion efficiency. But in order to increase the width of the depletion layer it is necessary to correspondingly increase the reverse bias voltage applied to the junction or diode. Further, in order to apply such high reverse bias voltage to the junction it is necessary to increase the specific resistances or resistivity of respective regions where the depletion layer is formed, which results in an increase of the internal resistance of the diode.
When the received light penetrates deeply into the APD as described above, carriers are formed by the light that reached regions other than the depletion layer and such carriers diffuse to reach the depletion layer and superpose upon the current component formed by the light that has reached the depletion layer. The transit time of the carriers in the depletion layer is extremely short because, due to high electric field in the depletion layer, the carriers reach high field regions near the junction in a short time and at a high saturation speed (10.sup.6 -10.sup.7 cm/sec) thus undergo avalanche multiplication. On the other hand, in the regions other than the depletion layer where electric field is not impressed, the carriers formed in these regions migrate towards the depletion layer due to the diffusion effect. For this reason, the time required for the carriers in the regions other than the depletion layer to reach the depletion layer is relatively long with the result that the current will continue to flow after end of the received light pulse. This degrades the response characteristic of the diode.